Do execGrouping.c via expression eval machinery.

This has a performance benefit on own, although not hugely so. The
primary benefit is that it will allow for to JIT tuple deforming and
comparator invocations.

Author: Andres Freund
Discussion: https://postgr.es/m/20171129080934.amqqkke2zjtekd4t@alap3.anarazel.de

src/backend/executor/nodeRecursiveunion.c

@@ -32,11 +32,14 @@ static void
 build_hash_table(RecursiveUnionState *rustate)
 {
 	RecursiveUnion *node = (RecursiveUnion *) rustate->ps.plan;
+	TupleDesc	desc = ExecGetResultType(outerPlanState(rustate));
 
 	Assert(node->numCols > 0);
 	Assert(node->numGroups > 0);
 
-	rustate->hashtable = BuildTupleHashTable(node->numCols,
+	rustate->hashtable = BuildTupleHashTable(&rustate->ps,
+											 desc,
+											 node->numCols,
 											 node->dupColIdx,
 											 rustate->eqfunctions,
 											 rustate->hashfunctions,